In the middle of Blur's triumphant performance at Glastonbury last year, Damon Albarn fell to his knees in tears after singing the band's hit To the End, overcome by the emotional force of the moment. As he walked off stage after a euphoric set, he had no plans to return.

Today, in an unexpected move from festival organisers, it can be revealed that Albarn is to make an unscheduled return to the Pyramid stage this month, when his band Gorillaz headline on Friday night.

The band were drafted in at the 11th hour after rock legends U2 cancelled their appearance after a severe back injury left singer Bono unable to perform.

Michael Eavis, who founded the festival which celebrates its 40th anniversary this year, said he was delighted Gorillaz were willing to step into the breach.

Eavis said: "This is going to be Gorilllaz's only UK festival appearance, and it'll be a massive audio visual spectacle which will really ignite the Pyramid on the Friday night, with Muse, then Stevie Wonder to follow.

"I'm very excited about Gorillaz' show coming here because they're so open to guests and collaborations. The alchemy of Friday's show is going to be astonishing, a perfect, contemporary way to kick off the 40th anniversary celebrations.

"I am so grateful for the enthusiasm of the media and the whole of the music industry for their willingness and eagerness to support us in what could have been a crisis."

Having started life as a semi-anonymous "virtual" band, the members have cartoon alter-egos created by one of their number, the graphic artist Jamie Hewlett.

In a statement to the Guardian, Murdoc, the band's animated bassist, styled the band as the festival's heroic saviours. "We're like some great big horrible warship pulling in to the bay of Glastonbury to save the day. It was us or the Beatles and they split up years ago," he said.

"The previous soldiers got pulled from duty last minute so it's up to my Plastic Beach naval cavalry to sail in and sort the battlefield out. I can assure you though, I'm bringing extra troops. Loads of them. Glastonbury will be ours … cutlasses drawn, trumpets ready. We're coming in …"

The news will surprise thousands of Glastonbury fans who have been in a state of heightened anticipation since the U2 cancellation was announced.

Names such as Led Zeppelin, Coldplay and the Rolling Stones had been mooted as possible replacements, with others backing pop star Kylie Minogue who cancelled her headline slot in 2005 after she was diagnosed with breast cancer. Others suggested that grime artist Dizzee Rascal, due to play the set before U2, would step up to the mark.

But Albarn's project, although suggested by a few of those in the know, had received little attention. Albarn, the creator of grand Chinese operas and purveyor of African adventures, was too fresh in the memory, too close to be considered.

The band are a difficult act to define. Started as a musical and animation project in 1998 when Albarn joined forces with Hewlett, co-creator of the comic book Tank Girl, for many years they recorded and "appeared" as a virtual band. Performers were hidden behind screens and represented only by their animated characters, lead vocalist 2D, bassist Murdoc, guitarist Noodles and drummer Russel Hobbs. Their music, too, defies classification – touching on hip hop, indie, dub, electronica and pop.

But recent performances, promoting new album Plastic Beach, saw a range of widely-respected musicians join Albarn on stage unshackled from their personas. Videos and filmic animations played out on a wide screen at the rear of the stage, while performances from an array of stars inspired alloyed joy from critics and fans.

At the Roundhouse in London last month, Albarn paraded his musical muscle, calling on the Clash's Mick Jones and Paul Simonon, as well as Mos Def, Bobby Womack, De la Soul and Shaun Ryder, to join him on stage.

The festival would not speculate on which special guests Albarn, Gorillaz's only constant musician, was likely to bring to Glastonbury's biggest ever party, but the indications were that this year's Glastonbury go-ers will not be disappointed.

The Guardian is the official media sponsor of Glastonbury festival, which takes place on 25-27 June. All tickets have sold out.